Light sources, as are used in a vehicle, for example, require a certain operating voltage range. Thus, for example, a voltage in a range of from 8 V to 16 V enables the operation of a light source at full brightness. Furthermore, however, operation of the light source at reduced brightness in a voltage range of from 6 V to 8 V may also be required.
In the case of the series circuit of a plurality of light-emitting diodes (LEDs), a forward voltage is typically between 6 V and 8 V. In order to adjust this forward voltage, a step-down converter can be used, starting from the operating voltage mentioned by way of example in the range of from 8 V to 16 V.
However it is problematic that such a series circuit of LEDs already illuminates at insufficient brightness at an operating voltage of 6 V, for example. The reason for this consists in that the luminous flux of the light-emitting diode is already too low at an operating voltage of 6 V owing to the exponential diode characteristic in conjunction with the mentioned step-down converter. In such a case, i.e. during operation of the light-emitting diode with reduced brightness at the predetermined operating voltage of 6 V, a step-up converter could be provided in order to raise the operating voltage again such that the light-emitting diode illuminates with sufficient brightness. Instead of the above-mentioned step-down converter, a step-up converter could therefore be used. In this case, it is disadvantageous that such a measure increases the cost of the operating device of the light source and also increases the size thereof owing to the additionally required component parts.